Phase locked arrays of antiguide laser structures were demonstrated over a decade ago. See, D. Botez, et al. “High-Power Diffraction Limited Beam Operation from Phase Locked Diode Laser Arrays of Closely Spaced ‘Leaky’ Wave Guides (Antiguides),” Appl. Phys. Lett., Vol. 53, 1988, pp. 464 et seq. In an antiguide laser, the antiguide core has an index no lower than the cladding index, n1. Whereas in a positive index guide, light is trapped in the guide core via total internal reflection, in an antiguide light is only partially reflected at the antiguide-core boundaries. Light refracted into the cladding layers is radiation leaking outwardly with lateral (projected) wavelength λ1, and can be thought of as a radiation loss, αr. See, D. Botez, “Monolithic Phase-Locked Semiconductor Laser Arrays,” Chapter 1 in Diode Laser Arrays, D. Botez and D. R. Scifres, Eds. Cambridge, U.K., Cambridge Univ. Press, 1994, pp. 1-72. For a proper mode to exist, αr has to be compensated for by gain in the antiguide core. The effective indices of the supported leaky modes have values below the core index, the quantum-mechanical equivalent being quasi-bound states above a potential barrier. Although in a single antiguide the radiation losses can be quite high, closely spacing antiguides in linear arrays significantly reduces the device losses, since radiation leakage from individual elements mainly serves the purpose of coupling the array elements.
Due to lateral radiation, a single antiguide, for which the index of refraction varies in only one dimension, can be thought of as a generator of laterally propagating traveling waves of wavelength λ1. In an array of antiguides, elements will resonantly couple in-phase or out-of-phase when the interelement spacings correspond to an odd or even number of (lateral) half-wavelengths λ1/2, respectively. When the resonance condition is met, the interelement spacings become Fabry-Perot resonators in the resonance condition, and thus full transmission occurs through the array structure allowing each element to equally couple to all others (i.e., global coupling is achieved). Resonant leaky-wave coupling allows the realization of global coupling for any type of phase locked laser array. See, D. Botez, et al. “Resonant Leaky-Wave Coupling in Linear Arrays of Antiguides,” Electron. Lett., Vol. 24, August 1988, pp. 1328-1330; D. Botez, et al. “Resonant Optical Transmission and Coupling in Phase-Locked Diode Laser Arrays of Antiguides: The Resonant Optical Waveguide Array,” AppI. Phys. Lett., Vol. 54, May 1989, pp. 2183-2185. At its resonance, the in-phase mode intensity profile becomes uniform. The unwanted out-of-phase mode(s) are non-resonant, which causes their fields to be significantly trapped between elements, and these modes can thus be effectively suppressed using interelement loss. Another way of suppressing out-of-phase modes is by the use of intracavity Talbot-type spatial filters. See, D. Botez, et al., “Phase-Locked Arrays of Antiguides: Modal Content and Discrimination,” IEEE J. Quantum Electron., Vol. 26, March 1990, pp. 482-485. However, the above description for resonant leaky-wave coupling holds only for structures in which the index of refraction varies periodically in only one dimension, the lateral one. For real antiguided devices, the index of refraction varies periodically in two dimensions, such that resonant coupling of the elements does not necessarily occur when the interelement spacings correspond to an odd or even number of (lateral) half-wavelengths λ1/2, respectively, but rather when the interelement spacings correspond to an odd or even number of (lateral) half-wavelengths λ1/2 plus a length which is a function of the two-dimensional details of the structure. See, D. Botez, “Monolithic phase-locked semiconductor laser arrays,” pp. 1-71 in “Diode Laser Arrays,” D. Botez and D. R. Scifres eds., Cambridge Univ. Press, UK, 1994.
Edge emitting devices, called ROW arrays, have exceeded the one watt coherent-power barrier (D. Botez, et al., “Watt-Range, Coherent Uniphase Power from Phase-Locked Arrays of Antiguide Diode Lasers,” Appl. Phys. Lett., Vol. 58, May 1991, pp. 2070-2072), demonstrated 10 W of peak pulse power in a beam twice the diffraction limit (H. Yang, et al., “10 W Near-Diffraction-Limited Pulsed Power From 0.98 μm-Emitting, Al-Free Phase Locked Antiguided Arrays,” Electron. Lett., Vol. 33, 1997, pp. 136-138), and 1.6 W continuous wave (CW) power in a twice diffraction limited beam (H. Yang, et al., “1.6 W Continuous-Wave Coherent Power From Large-Index-Step [Δn≈0.1] Near-Resonant Antiguided Diode Laser Arrays” Appl. Phys. Lett., Vol. 76, 2000, pp. 1219-1221). These milestones in stable, coherent power were due both to global coupling as well as to high built-in index steps (Δn=0.05-0.10) structures, which makes the desired in-phase mode relatively insensitive to gain spatial hole burning (GSHB) and thermal lensing. Comprehensive above-threshold analyses have confirmed the basic immunity of ROW arrays to GSHB. Furthermore, unlike evanescent-wave-coupled arrays, ROW arrays do not display coupling-induced instabilities, as expected for globally-coupled arrays. ROW arrays, due to large index steps as well as reliance on periodic gain modulation for selecting lasing of specific traveling-wave modes, were effectively the first active photonic lattices (APLs) employed for the generation of high coherent power. Bloch-function analysis showed them to be equivalent to 2nd order complex-coupled lateral distributed feedback (DFB) structures of zero stopgap, and further Bloch-function analyses of finite structures have allowed the derivation of analytical formulae for all relevant design parameters.
Antiguided-array structures have also been used for creating other APL-type devices. These include flat phasefront, stable beam fanout MOPA devices (See, Zmudzinski, et al., “3-Core ARROW-Type Diode Laser: Novel High-Power Single-Mode Device, and Effective Master Oscillator for Flared Antiguided MOPAs,” IEEE J. Select. Topics Quantum Electron., Vol. 1, No. 2, June 1995, pp. 129-137; D. Botez, et al., “Flat Phasefront Fanout-Type Power Amplifier Employing Resonant-Optical Waveguide Structures,” Appl. Phys. Lett., Vol. 63, December 1993, pp. 3113-3115), ARROW devices (L. J. Mawst, et al., “Design Optimization of ARROW-Type Diode Lasers,” IEEE Photonics Tech. Lett., Vol. 4, November 1992, pp. 1204-1206; L. J. Mawst, et al., “High-Powered, Single Mode, Al Free InGaAs, [P]/InGaP/GaAs Distributed Feedback Diode Lasers,” Journal of Crystal Growth, Vol. 195, 1998, pp. 609 et seq.; D. Zhou, et al., “Simplified Antiresonant Reflecting Optical Wave Guide-Type Vertical-Cavity Surface-Emitting Lasers,” Appl. Phys. Lett., Vol. 76, 2000, pp. 1659 et seq.); and Triple-Core ARROW Devices (A. Bhattacharya, et al., “0.4 W CW Diffraction-Limited-Beam Al-Free, 0.98 μm Three Core ARROW-Type Diode Lasers,” Electron. Lett., Vol. 32, 1996, pp. 657-658) which have demonstrated high CW (≧0.4 W) coherent powers, as well as well as one-dimensional and two-dimensional ROW arrays of vertical cavity surface emitting lasers (VCSELs) (S. K. Serkland, et al., “Two-Element Phased Array of Antiguided Vertical-Cavity Lasers,” Appl. Phys. Lett., Vol. 75, 1999, pp. 3754 et seq.; D. Zhou, et al., “Two-Dimensional Phase-Locked Antiguided Vertical Cavity Surface-Emitting Laser Arrays,” Appl. Phys. Lett., Vol. 77, 2000, pp. 2307 et seq.). However, ROW arrays can be prone to self-pulsations either if saturable absorption occurs in lossy interelement regions or when imaging, in intracavity Talbot-type spatial filters, is disturbed by GSHB. Single-frequency pulsed operation can be achieved for edge-emitting devices by using DFB gratings, but the yield of in-phase-mode operating devices has been quite low, since the effective yield is a strong function of the grating phase(s) with respect to the cleaved mirror facet(s). M. P. Nesnidal, et al., “Distributed Feedback Grating Used as an Array-Mode Selector in Resonant Antiguided Diode Laser Arrays: Effects of the Mirror Facet Position With Respect to the Grating,” IEEE Photon. Tech. Lett., Vol. 10, 1998, pp. 507 et seq.; and N. Nesnidal, et al., “0.45 W Diffraction-Limited-Beam and Single-Frequency Operation from Resonant Antiguided Phase-Locked Laser Array With Distributed Feedback Gratings,” Appl. Phys. Lett., Vol. 73, 1998, pp. 587 et seq.
Second-order DFB laser structures for use as surface emitters, based on outcoupling perpendicular to the chip surface, have been studied for nearly three decades. However, it has been found both theoretically as well as experimentally that the favored mode to lase is an antisymmetric one (that is, a two-lobed pattern), since it has the least radiation loss. Furthermore, the guided-field pattern is highly nonuniform, making the device vulnerable to multimoding via longitudinal GSHB. C. H. Henry, et al., “Observation of Destructive Interference in the Radiation Loss of Second-Order Distributed Feedback Lasers,” IEEE J. QE, Vol. 21, 1985, pp. 151-153.
Several approaches have been tried to obtain symmetric-like mode operation or actual symmetric mode operation. The first approach involves using a π phase-shifting film deposited on half the device aperture (S. H. Macomber, et al., “Recent Developments in Surface Emitting Distributed Feedback Arrays,” Proc. SPIE, Vol. 1219, 1990, pp. 228 et seq.). The second approach involves a long (about 2 mm) chirped grating (S. H. Macomber, “Nonlinear Analysis of Surface-Emitting Distributed Feedback Lasers,” IEEE J. QE, Vol. 26, 1990, pp. 2065-2074), which phase shifts the antisymmetric mode such that the devices operate in an off-normal single lobe pattern. A third approach causes pure symmetric-mode operation either by preferential carrier injection in a weak-coupling grating region (N. W. Carlson, “Mode Discrimination in Distributed Feedback Grating Surface Emitting Lasers Containing a Buried Second Order Grating,” IEEE J. QE, Vol. 27, 1991, pp. 1746-1752), or by introducing a metal grating which suppresses antisymmetric-mode lasing (M. Kasraian, et al., “Metal Grating Outcoupled, Surface-Emitting Distributed Feedback Diode Lasers,” Appl. Phys. Lett., Vol. 69, 1996, pp. 2795-2797). However, preferential carrier injection is not a long-term reliable approach, and the scheme, due to the necessity for weak coupling grating, inherently leads to inefficient devices (about 10% efficiency). The metal-grating variant of the approach is feasible but introduces too much of a penalty loss for the symmetric mode, such that efficiencies are at best 25-30%, and the gain thresholds are quite high (about 70 cm−1).
More recently, a solution to obtaining a symmetric-mode beam pattern with no penalty in device efficiency has been found in the use of central grating phase shifts of around π in distributed feedback/distributed Bragg reflector (DFB/DBR) devices. G. Witjaksono, et al., “Surface-Emitting Single Lobe Operation from 2nd-Order Distributed-Reflector Lasers With Central Grating Phase Shift,” Appl. Phys. Lett., Vol. 78, 2001, pp. 4088-4090; Dan Botez, et al., “Single Mode, Single Lobe Surface Emitting Distributed Feedback Semiconductor Laser,” Published International Application No. WO 01/13480 A1, 22 Feb. 2001. An example is a structure having a double-quantum-well (DQW) InGaAs/InGaAsP active region with InGaP cladding layers, and a grating formed in a P+-GaAs cap layer. The DQW active region is designed to be 0.4-0.5 μm away from the metal contact such that the device efficiency and reliability are unaffected. A symmetric-like mode is favored to lase over the antisymmetric-like mode when the grating phase shift, Δφ, ranges from 100° to 280°, with maximum discrimination occurring when Δφ=180°, i.e., a half wave (λ/2) central phase shift. The 180° phase shift does not affect the in-plane propagating (guided) light, as the field round trip through the phase shifter is 360° (i.e., the guided field remains antisymmetric). For the same reason, the 180° phase shift region does not affect the DFB/DBR grating, since the lasing occurs at the same wavelength, close to the Bragg wavelength, with or without a 180° phase shift. That is, the 180° phase shift creates no defect in the active photonic lattice. However, for the grating-outcoupled light, the 180° central phase shift region defines two surface emitting regions whose outcoupled fields are out-of-phase with each other, and thus the outcoupling of the guided antisymmetric field provides in-phase (i.e., symmetric) radiated near-field and far-field patterns. These types of devices also allow for relatively large tolerances in device fabrication, providing a practical solution for single (orthonormal)-lobe efficient surface emission from 2nd-order DFB lasers.
For devices optimized for maximum external differential quantum efficiency, ηd, the variation of the threshold gain and ηd have been studied as a function of the grating duty cycle, σ, defined as the ratio of Au as part of the grating period. G. Witjaksono, et al., “High-Efficiency, Single-Lobe Surface Emitting DFB/DBR Lasers,” Paper TuA3, 14th IEEE LEOS. Annual Meeting, 12-15 Nov. 2001, San Diego, Calif. The intermodal discrimination, Δα, reaches a maximum 113 cm−1 for σ=0.5, while the symmetric mode (S-mode) threshold gain is only 22 cm-−1 for σ=0.4, with a respectable Δα value of 52 cm−1. In general, it is found that such devices can tolerate some variation in grating duty cycle at a relatively small penalty in slope efficiency.
Gratings with phase shifts can be patterned by e-beam lithography or by holographic exposure of side-by-side negative and positive resists. However, current e-beam lithography allows writing of gratings only 400-600 μm long, and for devices requiring relatively long gratings (e.g., about 1,500 μm), fabrication by e-beam lithography is not advisable. The holographic method has been used to fabricate 1st-order gratings with quarter-wave (i.e., π/2) phase shifts, with the transition from negative to positive resists creating a grating phase shift of half the grating period. Using the same method for 2nd-order gratings naturally provides half-wave (i.e., π) phase shifts. Semiconductor (GaAs) gratings with π phase shifts have been developed using negative and positive resists (G. Witjaksono, et al. paper, TuA3, supra). A transition region is observed, but its width is not that relevant as long as the two grating regions are out-of-phase with each other. That is, the grating phase shift does not necessarily have to be π; it can be an odd number of π, since the in-plane propagating (guided) light is unaffected by it.
Two-dimensional (2-D) single-mode, single-lobe surface emitters (horizontal resonant cavity) are ideal high-power (≧1 W) coherent sources due both to low aspect ratio beams as well as the potential for scaling up the power by the use of coherent coupling of the sources at the wafer level (i.e., monolithically). L. J. Mawst, et al., “2-D Coherent Surface-Emitting Leaky Wave Coupled Laser Arrays,” IEEE J. Quantum Electron, Vol. 29, 1993, pp. 1906-1917. Three such types of devices have been reported. One involves angled gratings, K. N. Dzurko, et al., “Distributed Bragg Reflector Ring Oscillators: Large Aperture Source of High Single Mode Optical Power,” IEEE J. Quantum Electron., Vol. 29, 1993, pp. 1895-1899; M. Fallahi, et al., “Low Threshold CW Operation of Circular-Grating Surface-Emitting DBR Lasers Using MQW and a Self-Aligned Process,” IEEE Photon. Tech. Lett., Vol. 6, 1994, pp. 1280-1282. The third approach uses a curved-grating unstable resonator, S. H. Macomber, et al., “Curved-Grating Surface-Emitting DFB Lasers and Arrays,” Proceedings Society of Photo-Optical Instrumentation Engineers,” Vol. 3001, 1997, pp. 42-54. However, none of these devices have a built-in dielectric structure for lateral-optical-mode control and stability, and as a result are vulnerable to temperature and carrier induced dielectric-constant variations. An example of such behavior is the unstable resonator device which, while operating single-mode to high peak pulsed power in a single off-normal beam, readily becomes multimode in CW operation due to thermal lensing.